Pinched-off, resealed nerve terminals (synaptosomes) will serve as an in vitro model system for a multidisciplinary study of the physiology of presynaptic nerve terminals in the mammalian central nervous system. The following aspects of synaptosome function will be studied: 1. The actions of neurotoxins that affect the sodium channel inactivation mechanism will be studied by monitoring synaptosome membrane potentials with a voltage-sensitive fluorochrome. The binding of radioactive tracer-labelled neurotoxins to sodium channels synaptosome plasma membranes will also be measured. 2. The binding and selectivity properties of voltage-sensitive calcium channels will be studied by measuring depolarization-induced influx of 45Ca and other alkaline earth cations. Competition between the cations, and the effects of blocking cations (e.g., Co and Mn) and pH will also be tested. 3. Calcium fluxes will be measured in resealed synaptosome plasma membrane vesicles. The effects of Na ions and of ATP will be studied, in an effort to determine the source of energy for the maintenance of a 10 to the 4th power-fold Ca gradient across the plasmalemma. 4. Freeze-fracture and conventionally thin-section electron microscopic methods will be used to study the morphological correlates of neurotransmitter release (in synaptosomes): vesicle-plasmalemma-fusion, and vesicle membrane retrieval and recycling. 5. Complement fixation renders synaptosomes porous, so that small ions and molecules (e.g., Ca, Mg, ATP) can readily cross the plasmalemma. These leaky synaptosomes will be used to determine the internal conditions required for transmitter release. The aforementioned experiments should provide new and important information about some of the crucial aspects of presynaptic nerve terminal function.